This invention relates to an injection mold for manufacturing a two-color molding of synthetic resin.
In general, a magnetic tape cassette used for audio or video devices comprises a case body made up of upper and lower cassette halves, a pair of hubs rotatably supported in the case body and a magnetic tape wound on the hubs.
The upper and lower cassette halves are formed by injection molding plastic material or the like. More specifically, in order to form the upper or lower halves, molten material is injected through a direct gate and a submarine gate into the cavity which is formed between two metal molds in correspondence to the configuration of the cassette half. The molding of the cassette half is accomplished when the material thus injected has been cooled and hardened. The cassette half thus formed is taken out of the molds. Thereafter, printed design sheets are bonded to the outer surfaces of the cassette halves or designs are printed directly on them.
In order to allow the user to observe the magnetic tape in the cassette from outside, the upper and lower halves are made of transparent material, or as shown in FIG. 4, a transparent or translucent window 6 is formed in the cassette half body 2a which is opaque. The window 6 may be modified in various ways to improve the design of the case body 1.
Heretofore, to provide the window 6, a window member is formed separately from the cassette half body 2a, and then joined to the latter with adhesive or by ultrasonic welding. In this case, the design is limited because it is necessary to form the window member and to bond it to the cassette half body. In addition, the method is low in productivity. Hence, recently, a method of manufacturing a magnetic tape cassette by two-color molding using a core back system has been employed.
A method of manufacturing the cassette halves according to the core back system will be described with reference to FIGS. 5 and 6, which are sectional views of a cassette half mold which corresponds to the section A--A of the audio compact cassette shown in FIG. 4. The metal injection mold according to the core back system, as shown in FIG. 5, includes a stationary mold 20, a movable mold 22, a slide core 26 fitted in a slide hole 24 formed in the stationary mold 20, a slide core 30 fitted in a slide hole 28 formed in the movable mold 22, and a stationary block 25 provided in the slide core 26. A mold cavity 32 for forming the above-described cassette halves 3a and 3b is defined by the facing inner surface of the stationary mold 20 and the movable mold 22. The slide cores 26 and 30 protrude into the cavity 32 in such a manner that they abut against each other, to form the above-described window 6.
The cavity 32 communicates with a submarine gate 33 which is formed in the movable mold 22 in order to inject primary resin, and with a direct gate 34 which is formed in the stationary mold 20 near the slide core 26 in order to inject secondary resin (in the case of FIGS. 5 and 6, the direct gate 34 is provided in the stationary block 25).
To mold the cassette half body 2a, first the stationary mold 20 and the movable mold 22 are brought together. Thereafter, the slide cores are inserted into the cavity with hydraulic drive means until they abut against each other substantially at the middle of the thickness of the cavity 32. Under these conditions the opaque primary resin is injected into the cavity through the submarine gate 33. Then, as shown in FIG. 6, the slide cores 26 and 30 are retracted to form a window cavity 36. Finally, the transparent secondary resin is injected into the window cavity 36 through the direct gate 34 provided in the stationary block 25 to form the window 6.
The cassette half body 2a and the window 6 are welded together by the heat and pressure provided during the injection molding. The slide cores 26 and 30 are different in dimensions, and therefore a step is formed in the window 6, thus making the welded portion of the window 6 and the cassette half body 2a strong enough to withstand the force applied in the direction of thickness.
As described above, since the direct gate 34 is provided near the slide core 16, the slide core 26 is liable to be thermally expanded by the heat of the injected molten resin. Further, the outer wall of the end portion of the slide core 26 protruding from the slide hole 24 is not laterally supported by anything when positioned as shown in FIG. 5. Therefore, the expansion of the slide core 26 may impede its accurate sliding retraction to form the window cavity. To minimize this problem it is necessary to manufacture the stationary mold 20 and the slide core 26 with a high degree of accuracy, which is costly.
Further, the molten resin injected under pressure into the window cavity 36 may creep between the already-formed opaque portion of the cassette half body 2a and the stationary mold 20, thus deforming the cassette half body and/or impairing its external appearance.